1. Field of the Invention
The present invention relates to a method and system for detecting the presence of pathogenic microorganisms in a sample. More specifically, the present invention relates to a method and system for rapidly detecting and identifying target pathogenic microorganisms in a sample that involves the use of a capture matrix throughout the resuscitation, growth and detection processes.
2. Background of the Technology
Each year, millions of persons experience food borne illness caused by pathogenic microorganisms. Only a fraction seeks medical care and even a smaller number submit laboratory specimens. Food borne illness is a continuing threat to all persons, regardless of age, race, sex, lifestyle, ethnic background and socioeconomic status. Food borne illness causes suffering and death and imposes a financial burden on society. The annual national cost associated with food borne illness ranges from 2.9 to 6.7 billion dollars. This range is mainly for the combined direct and indirect costs associated with food borne illness caused by six pathogens: Campylobacter jejuni or Campylobacter coli; Clostridium perfringens; E. coli O157:H7; Listeria monocytogenes; Salmonella (non-typhoid); and Staphylococcus aureus. 
It has long been recognized that the potential for pathogenic microorganism (pathogen) contamination exists in products which are designed for human consumption or use.
These pathogens are generally present in the colon, intestines or fecal matter of humans or animals. When food products such as poultry, red meat, seafood, eggs or any foods which contain these products, come into contact with fecal matter during handling or processing, the potential exists for contamination and subsequent transfer of the pathogens to the end user.
Not only can the presence of sufficient numbers of pathogens cause the deterioration or spoilage of a food product, such can also cause disease if consumed by a human or animal. While most human cases of bacterial food poisoning only result in acute symptomatic disease, e.g., nausea, vomiting, diarrhea, chills, fever and exhaustion; for individuals such as infants, the elderly, pregnant women, neonates and the immunocompromised, death can occur.
In order to prevent the transmission of food borne pathogens, the manufacturers and/or processors of food products routinely test samples of their products as well as processing equipment to identify contamination before placing a product into the stream of commerce. Typically, pathogens such as enteric bacteria, i.e., Enterobacteriaceae, such as Salmonella and E. coli, and Gram positive organisms, such as Staphylococcus and Enterococcus, are the organisms tested for. Because pathogens can be present in very small numbers in a food product which may contain a large number or variety of other non-pathogenic microorganisms, methods for the recovery and detection of pathogens have been developed.
Pathogens are typically injured during processing of products. That is, the pathogens may have undergone heating, freezing, contact with chemical additives, or mechanical processing steps which injure or debilitate the pathogens. Thus, resuscitation of the pathogens before detection is often required.
A primary technique for screening for the presence of pathogens involves the use of a series of media (i.e., nutrient fluid) transfers starting from the use of a non-selective enrichment medium, and then the use of a selective medium. The non-selective enrichment medium is usually employed to resuscitate potentially injured pathogens. Once the pathogens have been revived, a small quantity of the non-selective enrichment medium is then transferred into the selective medium. This technique, which is often defined by human work patterns and the growth patterns of the pathogens, generally takes several days to complete.
For example, culturing a food sample for the presence of Salmonella typically involves the addition of approximately 20-25 grams (g) of sample, such as meat, into approximately 225 milliliters (ml) of non-selective enrichment medium, e.g., Buffered Peptone Water (BPW) or Universal Pre-enrichment Broth (UPB), to allow for repair of injured microorganisms. The sample is thoroughly mixed with non-selective enrichment broth and incubated for up to about 22-28 hours at approximately 35 C+/−2 C. Following this step, the sample is further selectively enriched in a growth promoting medium containing inhibitors that allow for the continued growth of a target pathogen, while simultaneously restricting the proliferation of most other competing microorganisms, and further incubated for up to an additional 22-28 hours at approximately 35 C. This secondary enrichment step is generally followed by a detection step that can include plating the secondary enrichment broth onto solid media or by utilizing other methods, such as the use of specific enzyme markers, biochemical testing, immunological assays or DNA probes.
Over the years, many attempts have been made to shorten the length of the primary enrichment step to reduce total assay time. Generally, it has been found that at least 6 to 12 hours of non-selective enrichment is required to obtain enough viable pathogens for further testing.
U.S. Pat. No. 5,145,786 discloses a five step method for the recovery of pathogens. The first step is a pre-enrichment, wherein the food sample is enriched in a non-selective medium to restore injured pathogenic cells to a stable physiological condition. The second step involves a selective enrichment wherein inhibitory reagents are added to the growth medium to promote the growth of the selected pathogens while restricting the proliferation of most other bacteria. The third step involves plating a sample of the enriched growth medium onto a solid selective media to physically isolate pure, discrete colonies of the suspected pathogen. The fourth step involves biochemical testing or screening of the pure cultures obtained from the selective plating step in order to eliminate non-target or competing organisms. The fifth step involves a serological analysis of the pure culture of suspected pathogen in order to specifically identify pathogen(s) present in the sample A major disadvantage of this method, which takes from 3 to 6 days to complete, is that it is very labor intensive and time consuming. This lengthy analysis is often not suitable for producers of perishable products because the producers must keep the products from consumers until the test period is over.
In order to reduce the amount of time for detection, other methods have been developed. The REVEAL® Salmonella Test System (Neogen Corp., Lansing, Mich., U.S. Pat. No. 5,296,370) shortens the time frame necessary to approximately two days. On day one, a sample to be tested is inoculated into a non-inhibitory pre-enrichment medium. After approximately two to four hours of incubation, highly selective inhibitors are added to enrich only for specific bacterial pathogens while inhibiting the proliferation of other microorganisms present in the sample. On day two, the samples are assayed by specific immunoassay in order to detect positive and negative samples. All positive samples are then tested further in order to confirm their identity.
U.S. Pat. No. 5,843,699 discloses a method for the detection and identification of target pathogens in a sample which may contain both target pathogens and competing non-target microorganisms. The method involves liquid enrichment of a sample to be tested in order to adjust the levels of the target pathogens in the enrichment medium to be at or above the level of other microorganisms and subsequent biochemical identification.
U.S. Pat. No. 5,296,370 discloses a repair medium that includes a non-selective nutrient medium supplemented with one or more antioxidants, a yeast derivative, an agent capable of reducing oxygen tension in the medium and one or more fatty acids.
U.S. Pat. No. 5,100,801 discloses an apparatus for selective microbial enrichment. The apparatus may include a means for screening liquid between a collection chamber and container. The means for screening acts as a barrier for large particulate materials but not for the target pathogens.
U.S. Pat. No. 5,145,786 discloses a pre-enrichment broth that comprises tryptone, proteose peptone, glucose, sodium pyruvate, ferric ammonium citrate, sodium chloride, magnesium sulfate and buffer salts.
U.S. Pat. No. 5,411,867 discloses a low pH media that contains trimethylamine-N-oxide that promotes the growth of chlorine or food processed injured E. coli. The broth may be used in combination with known membrane filtration techniques, which involve capturing bacteria on a membrane filter and transferring the membrane filter to a petri dish containing the media to enable the bacteria to form large colonies.
U.S. Pat. No. 5,443,987 discloses a detection system that can be positioned in-situ in a health care product or can be associated with the health care product packaging such that microbial contamination can be detected.
U.S. Pat. No. 5,462,860 discloses a conditioned culture medium that includes gelatin, casein or animal peptones, yeast extract, carbohydrates, salts and indicators. The conditioned culture medium may be coated onto a self-supporting waterproof substrate, dried and inoculated with sample for detection of pathogen when an added indicator changes color in the presence of selected metabolites.
U.S. Pat. No. 5,510,243 discloses a method for the detection of E. coli in a sample. The method involves capturing the target bacteria on a filter membrane and placing the filter membrane in contact with a semi-solid nutrient medium on a solid support.
Although there are pathogen detection methods in existence, none of them provide a simple and rapid detection of pathogens. Thus, there remains a need for a rapid, sensitive, specific, user friendly, and cost effective method for pathogen detection